1. Field of the Invention
The present invention relates to a semiconductor device which includes a circuit including a semiconductor element such as a transistor, and a method for manufacturing the semiconductor device. For example, the present invention relates to a power device incorporated in a power circuit; a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display panel; a light-emitting display device including a light-emitting element; and an electronic device including any of the above as a component.
Note that in this specification, a semiconductor device means any device that can function by utilizing semiconductor characteristics. An electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A transistor formed over a glass substrate or the like is manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in a liquid crystal display device. Although a transistor including amorphous silicon has low field-effect mobility, it can be formed over a larger glass substrate. On the other hand, although a transistor including polycrystalline silicon has high field-effect mobility, it is not suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and such a transistor is used as a switching element or the like of a pixel of a display device.
Meanwhile, it has been pointed out that hydrogen is a source of carriers particularly in an oxide semiconductor. Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor at the time of forming the oxide semiconductor. Further, shift of the threshold voltage of a transistor including an oxide semiconductor is reduced by reducing the amount of hydrogen contained in not only the oxide semiconductor but also a gate insulating film in contact with the oxide semiconductor (see Patent Document 3).